1. Field of the Invention
The present invention relates to a membrane-electrode assembly for a fuel cell and a fuel cell system including the membrane-electrode assembly for a fuel cell. More particularly, the present invention relates to a membrane-electrode assembly for a fuel cell, which is more firmly combined and thereby can reduce cross-over of a fuel, and a fuel cell system including the membrane-electrode assembly for a fuel cell.
2. Related Art
A fuel cell is a power generation system for producing electrical energy through an electrochemical redox reaction of an oxidant with a fuel such as hydrogen or a hydrocarbon-based material such as methanol, ethanol, natural gas, or the like. Such a fuel cell is a clean energy source that can replace fossil fuels. It includes a stack composed of unit cells and produces various ranges of power output. Since it has four to ten times higher energy density than a small lithium battery, it is lightweight and can be used as a small portable power source.
Exemplary fuel cells include a polymer electrolyte membrane fuel cell (PEMFC) and a direct oxidation fuel cell (DOFC). The direct oxidation fuel cell includes a direct methanol fuel cell that uses methanol as a fuel. The polymer electrolyte fuel cell has an advantage of a high energy density and high power, but also has problems that hydrogen gas of the fuel cell needs to be carefully handled and the fuel cell requires accessory facilities such as a fuel reforming processor for reforming methane or methanol, natural gas or the like into hydrogen gas.
On the contrary, a direct oxidation fuel cell has a lower energy density than that of the gas-type fuel cell but has the advantages of easy handling of a liquid-type fuel, a low operation temperature, and no need for additional fuel reforming processors. Therefore, the direct oxidation fuel cell has been acknowledged as an appropriate system for a portable power source for small electrical equipments.
In the above-mentioned fuel cell systems, the stack that substantially generates electricity includes several to scores of unit cells stacked adjacent to one another, and each unit cell is formed of a membrane-electrode assembly (MEA) and a separator (also referred to as a bipolar plate). The membrane-electrode assembly is composed of an anode (also referred to as a fuel electrode or an oxidation electrode) and a cathode (also referred to as an air electrode or a reduction electrode) that are separated by a polymer electrolyte membrane.
A fuel is supplied to an anode and absorbed on catalysts of the anode, and thereafter is oxidized to produce protons and electrons. The electrons are transferred into a cathode via an out-circuit, while the protons are transferred into the cathode through the polymer electrolyte membrane. In addition, an oxidant is supplied to the cathode, and then the oxidant, protons, and electrons are reacted together on catalysts of the cathode to produce electricity along with water.
The above information disclosed in this background section is only for enhancement of understanding of the background of the invention and therefore, it should be understood that the above information may contain information that does not form the prior art that is already known in this country to a person or ordinary skill in the art.